This invention relates generally to a loop detector of the type used for inductively sensing the presence or passage of a vehicle on a roadway.
Inductive loop vehicle detectors are being used to an increasing extent to detect the passage or presence of road going vehicles of all kinds. However an ancillary detection requirement, which is becoming of increasing importance, is the ability not only to detect a specific class of vehicles such as buses and emergency vehicles within a given area but also to detect specific individual vehicles. For example it is highly desirable to detect an emergency vehicle such as an ambulance in order to achieve traffic priority for the vehicle by favourably controlling the traffic lights at intersections through which the vehicle must pass. It is also necessary, for example to afford to a specific vehicle access to an area, to be able to identify vehicles on an individual basis. This ability may, in addition, be used to bill the responsible person for the use of a toll road, for low enforcement reasons, or for a variety of other purposes.
Specific vehicles of the kind mentioned are equipped with transmitters and inductive loops, buried in the roadways, act as sensors which are responsive to the transmitted signals. The advantage of this system, over other radio-based systems, is that the transmitter must pass over the loop for detection to occur and when this happens the precise location of the vehicle is determinable.
Standard vehicle detection systems currently use one or more buried loops at predetermined locations, e.g. at intersections, or at the approaches to such locations, and the number of loops increases with the complexity of the installation location. It is expensive to install a set of loops and it should be noted that each set of loops requires additional maintenance and care to ensure its continued operation. It should be borne in mind that the loops and feeders constitute the weakest link within a detection system.
It may also not be possible to install an additional loop at a desired location, for sensing a transmitted signal, as an existing detector loop may occupy a desired position. Under these circumstances a compromise installation must be arrived at.
Yet another problem is that the frequency band allocated for the transmitted signals generally falls within the frequency band which has been allocated to vehicle detectors. The detector loop functions as a transmitting element and this can cause interference problems with the receiver loop.